DE 102 60 435 A1 describes a method for operating a drive-train of a hybrid vehicle, the drive-train disclosed therein comprises an internal combustion engine and an electric machine, with a clutch connected between the internal combustion engine and the electric machine. In addition the drive-train described therein comprises a transmission connected between the drive aggregate and a drive output of the drive-train. From this prior art it is known, when the motor vehicle is being operated purely under electric-motor power by the electric machine of the hybrid drive, that to start the internal combustion engine the clutch connected between the internal combustion engine and the electric machine is closed in order to drag-start the internal combustion engine of the hybrid drive by means of the electric machine of the hybrid drive.
DE 10 2006 034 934 A1 describes a further method for operating a drive-train of a hybrid vehicle, in which to start the internal combustion engine, again a clutch connected between the internal combustion engine and an electric machine is closed; in this case the closing of the clutch and thus the transmission from the electric machine to the internal combustion engine of the torque that serves to start the internal combustion engine is coupled in time with a downshift process in the transmission.